Hello,I had always been understood that the Polar Star is a perfect star to align a Dobson telescope.The fact is that the last summer the pointing accuracy was quite high having used the Polar Star how first alignment star, This autum-winter changing the second alignment star (I can not remember which I used in summer), the accuracy was decreased enough. Last night I made an initial alignment with my 16 " f5 using the known parameters of ALT REF +090 000 Auto adjust on, and selecting Polaris and Altair as alignment stars (theoretically two stars meet the assumptions of separation and position in the sky) and the accuracy was mediocre to the east area, and acceptable to the west area ( Altair situation ). At 21.00 pm when Capella was high enough in the sky, I decided to align this time using this star and Deneb, and the situation changed dramatically. precision greatly improved, most of the objects were at midfield of a 12 mm Plössl reticle eyepiece (169x and 0.30º), and from there everything went into the field of an Ethos 10mm (200X and 0'5º), even with objects close enough to the zenith (sensitive area for Dobson) . Every time I have more clear that the stars used for the alignment are critical for the accurate pointing, whether other combinations may seem that meet the same requirements. Any more experience in this regard? .

The RA of Polaris is 2h31m, close to the other fall/winter time stars you are choosing as a second alignment star. The more difference there is between RA's of the two alignments you choose, the better your alignment will be.

The last several months I have used Polaris and Alpheratz with good results, and last time (a couple of weeks back) I got good results using Polaris and Aldebaran.

Here are a few neat little tricks for ServoCAT users:

1) Chose your non-Polaris star FIRST. Reason: For 2-star alignment Argo will uses first-in first-out memory. So if you are not happy with the results from the first 2-star align, you can just pick a third star to align on and retain Polaris in the solution.

2) Defocus the star so that you are using the secondary shadow for alignment. Much easier!

3) Use a good illuminated reticle. I'm using the Celestron Micro Guide with excellent results.

4) Align the reticle east-west such that the star is drifting along the illuminated bar. Worth taking a few minutes for best precision.

5) Use the GOTO button on the ServoCAT hand controller for catching that exact moment the star is centered.

6) Move on to Polaris and take as much time as needed for precise centering.

Using a medium-powered eyepiece with a 18 arc minute field I am usually in the inner half of that field on GOTO jumps.

Peter:On the first alignment, I used Polaris (2h 31 m) and Altair (19h 51m), are not far enough?.Jeff,I'm using a 12 mm plössl with illuminated reticle, and how you well say I do:-Align the reticle east-west such that the star is drifting along the illuminated bar. Worth taking a few minutes for best precision.-Use the GOTO button on the ServoCAT hand controller for catching that exact moment the star is centered.-Move on to Polaris and take as much time as needed for precise centering. The next once I will do this, to take Polaris how second alignment star

It's not so much the right ascension as the azimuth difference. Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees). You also want a good altitude difference, without using a star that's too close to the zenith (should be less than about 80-degrees Alt) or too close to the horizon (should be more than about 15-degrees Alt).

It's also important to remember that any orthogonality issues with the scope's movement and pointing axes (Az/Alt/optical) will impact the scope's pointing accuracy as the scope is moved away from the alignment stars. It's also very common to have pointing accuracy issues near the zenith and horizon.

Finally, if you're using 4000 count encoders, your built-in accuracy is about 1/10th of a degree. So if you're consistently 1 or 2 tenths of a degree off-center, there's not much you can do that will guarantee better performance (there's always the possibility of a "lucky" alignment, where the errors add up in your favor and all your selected targets end up centered or nearly so).

Vic, I am using 10000 count encoders. You say:"It's also important to remember that any orthogonality issues with the scope's movement and pointing axes (Az/Alt/optical) will impact the scope's pointing accuracy as the scope is moved away from the alignment stars".Well, but these issues should be present regardless of the selected alignment stars, isn't it?."Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees)". According to this criterion the Polaris should not be an ideal alignment stars.Thank you.

I should pay more attention to the title of the thread. Your Argo will utilize those 10,000 count encoders to extrapolate the best possible pointing accuracy.

...these (orthogonality) issues should be present regardless of the selected alignment stars...

Yes, they are. My point was that there is more than just alignment star selection that can impact accuracy. Again, your Argo Navis is capable of modeling orthogonality issues utilizing a TPAS run, which can reduce the impact of the error.

"Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees)". According to this criterion the Polaris should not be an ideal alignment stars.

Polaris is (approximately) due north, so any star that is due south will have an azimuth difference of 180-degrees.

One thing that has troubled me with regards to performing a two-star alignment with Dobs is the shift in altitude of the entire telescope as you quickly engage the ServoCAT's altitude drive to the altitude bearing after the second star is aligned at the eyepiece. At the eyepiece, I defocus the star in a 50° afov eyepiece at ~200X to center the star more accurately. Since the second star is moving (not Polaris) across the eyepiece fov, quickly engaging the altitude drive is important to keep alignment accuracy. With my past Starmaster Dob's and currently with my StarStructure Dob, engaging the altitude bearing to the drive lifts the entire telescope a bit so that at the eyepiece it moves the second aligned star almost out of the field of view - approx 1/4 degree at 200X. This effects all following go-to actions by positioning objects off center by that same 1/4 degree, in addition to what ever else went wrong with the two-star alignment. The pointing errors are all cumulative.

It's not so much the right ascension as the azimuth difference. Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees). You also want a good altitude difference, without using a star that's too close to the zenith (should be less than about 80-degrees Alt) or too close to the horizon (should be more than about 15-degrees Alt).

It's also important to remember that any orthogonality issues with the scope's movement and pointing axes (Az/Alt/optical) will impact the scope's pointing accuracy as the scope is moved away from the alignment stars. It's also very common to have pointing accuracy issues near the zenith and horizon.

Finally, if you're using 4000 count encoders, your built-in accuracy is about 1/10th of a degree. So if you're consistently 1 or 2 tenths of a degree off-center, there's not much you can do that will guarantee better performance (there's always the possibility of a "lucky" alignment, where the errors add up in your favor and all your selected targets end up centered or nearly so).

Over the summer. I was getting good results with my two-star alignment using Polaris and Altair with GoTo objects between these two stars.

This is what I have noticed about ServoCAT and Argo Navis one my current Dob: that the GoTo operation will be most accurate for finding objects 'between' the two alignment stars chosen and that the two stars shouldn't be more than ~100° apart nor any of them too close to the horizon. The more the telescope is asked to find an object away from the area between the two alignment stars, the more pointing errors pile up until you are better off using your optical finder. If you choose alignment stars within one quadrant of the sky (<90°x90°) and decide to look at objects outside of that area perhaps in another quadrant, then it's best to perform a new two-star alignment with new stars that are well-positioned in that area. Because if this, my procedure in observing for the evening is by GoTo'ing to objects in a constellation-by-constellation basis so that finding the objects are well 'within' the two alignment stars chosen.

Peter:On the first alignment, I used Polaris (2h 31 m) and Altair (19h 51m), are not far enough?.Jeff,I'm using a 12 mm plössl with illuminated reticle, and how you well say I do:-Align the reticle east-west such that the star is drifting along the illuminated bar. Worth taking a few minutes for best precision.-Use the GOTO button on the ServoCAT hand controller for catching that exact moment the star is centered.-Move on to Polaris and take as much time as needed for precise centering. The next once I will do this, to take Polaris how second alignment star

One thing that has troubled me with regards to performing a two-star alignment with Dobs is the shift in altitude of the entire telescope as you quickly engage the ServoCAT's altitude drive to the altitude bearing after the second star is aligned at the eyepiece. At the eyepiece, I defocus the star in a 50° afov eyepiece at ~200X to center the star more accurately.

For 50-degree AFOV at 200X, TFOV = 0.25-degree (I normally use a 100-degree AFOV at 200X with TFOV = 0.5-degree).

Since the second star is moving (not Polaris) across the eyepiece fov, quickly engaging the altitude drive is important to keep alignment accuracy.

It's not necessary to engage the altitude drive to select the second alignment star--you can even enter from the hand pad. I normally do a quick "manual" goto on a known object to verify the two-star alignment before I engage the ServoCAT clutches.

With my past Starmaster Dob's and currently with my StarStructure Dob, engaging the altitude bearing to the drive lifts the entire telescope a bit so that at the eyepiece it moves the second aligned star almost out of the field of view - approx 1/4 degree at 200X.

That should be a little less than 1/8 degree (about 0.1-degree) for a 1/4-degree TFOV eyepiece. I see some similar, small movement with my StarStructure (it could be 0.1-degree), but after I've verified the target centering manually (as I described above), when I press the GoTo button on my hand pad the target always moves back to the center of the FOV as the Argo Navis read out re-zeroes on the target (this is all part of my normal Argo Navis/ServoCAT "setup and verify" procedure). I don't understand why your scope behaves differently, perhaps it's because you're engaging the altitude drive before you press enter on the Argo Navis? Or have I misunderstood your actual procedure?

...the GoTo operation will be most accurate for finding objects 'between' the two alignment stars chosen and that the two stars shouldn't be more than ~100° apart nor any of them too close to the horizon.

My experience has been a little different from yours. If I can get an "optimal" two-star alignment (optimal as described by Alt Az Align), I get "good" performance (usually within +/-0.1-degree) for most of the sky except near the zenith (usually +/-0.2- to 0.3-degree). This makes sense to me, since my secondary mirror isn't mechanically offset away from the focuser, which generates a 0.2-degree error close to the zenith (and a 0.4-degree "hole" at the zenith). If I get a "good" two-star alignment, I also find that 6 hours later targets in the "new" half hemisphere of sky are still +/-0.1- to 0.2-degrees from center.

I rarely realign on a third star after I have a "good" two-star alignment, but I do use the local sync to recenter on objects located in heavily populated areas (galaxy clusters, etc.). If my alignment suddenly loses accuracy (as it sometimes does after hours of multi-target observing)--I just reinitialize with a new two-star alignment.

To clarify my two-star align procedure, with both drives 'disengaged' I hand slew the Dob to each alignment star pushing the ENTER button in the Argo after each star is centered in the eyepiece. I use a higher power 1-1/4" 6-8mm eyepiece with a 50° narrower-field (i.e, Plossl or Ortho) to view these stars fully racked out and defocused until the star's shape practically touches the fov edge. You can do this with a bright 1st or 2nd mag star. It's after the second star is manually centered at the eyepiece and entered in the Argo, I engage both drives, choose an object, actuate a Go-To and confirm the alignment accuracy. BTW: it's Mike Zammit of StarStructure that advised me to use this defocused star at high power for centering the alignment stars at the eyepiece. It's an easy way to accurately center a bright star.

One thing that has troubled me with regards to performing a two-star alignment with Dobs is the shift in altitude of the entire telescope as you quickly engage the ServoCAT's altitude drive to the altitude bearing after the second star is aligned at the eyepiece. At the eyepiece, I defocus the star in a 50° afov eyepiece at ~200X to center the star more accurately.

For 50-degree AFOV at 200X, TFOV = 0.25-degree (I normally use a 100-degree AFOV at 200X with TFOV = 0.5-degree).

Since the second star is moving (not Polaris) across the eyepiece fov, quickly engaging the altitude drive is important to keep alignment accuracy.

It's not necessary to engage the altitude drive to select the second alignment star--you can even enter from the hand pad. I normally do a quick "manual" goto on a known object to verify the two-star alignment before I engage the ServoCAT clutches.

With my past Starmaster Dob's and currently with my StarStructure Dob, engaging the altitude bearing to the drive lifts the entire telescope a bit so that at the eyepiece it moves the second aligned star almost out of the field of view - approx 1/4 degree at 200X.

That should be a little less than 1/8 degree (about 0.1-degree) for a 1/4-degree TFOV eyepiece. I see some similar, small movement with my StarStructure (it could be 0.1-degree), but after I've verified the target centering manually (as I described above), when I press the GoTo button on my hand pad the target always moves back to the center of the FOV as the Argo Navis read out re-zeroes on the target (this is all part of my normal Argo Navis/ServoCAT "setup and verify" procedure). I don't understand why your scope behaves differently, perhaps it's because you're engaging the altitude drive before you press enter on the Argo Navis? Or have I misunderstood your actual procedure?

Victor,
Gary recommends about 30 degrees or more separation for the alignment stars (Az). Make sure they have different Altitudes (>10 degrees Alt). So better work algorithm Auto adjust On.
Although I have questions about the stars in the Polaris region. For some stars (Align 2 stars region Polaris Az>30 Alt>10, mode Az Alt) I get RAW RMS up to 1.5 degrees(!) some values with FIT RMS 3.8-4.0' (based on a sample of 30 stars).
I do not understand why.

"Polaris is (approximately) due north, so any star that is due south will have an azimuth difference of 180-degrees". Yes but the line drawn between the two alignment stars hasn`t to go necessarily through the zenith to meet this requirement. Both could fall in either hemisphere.
Vic, please, could you say me for this geographic coordinates 36°27'42"N and 5°43'26"W, to the 18'45h local time, the program Alt Az Align, what alignment stars recommended?
Peter, it is recommended to use ServoCAT goto button after each alignment star is centered in the eyepiece.

"ServoCAT goto button after each alignment star is centered in the eyepiece"
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I often do not work GoTo ServoCat -> signal Align Argo. I have two box ServoCat. Wired and Wi-Fi. Collisions? I do not use goto button for Align ArgoNavis.
Ps. Yellow buttons Up, Down etc work. Green (GoTo for Align) 50/50 work/not work. GoTo (for GoTo) - works (on both box).

Ditto for me. I've never heard of pushing the GOTO button on my Hand Controller after each align star is centered in the eyepiece and entered on the Argo. Is this a new direction to follow? If you push the GOTO button on the HC at that point in time, then the disengaged drives will start to spin up out of control.

"ServoCAT goto button after each alignment star is centered in the eyepiece"
______________________

I often do not work GoTo ServoCat -> signal Align Argo. I have two box ServoCat. Wired and Wi-Fi. Collisions? I do not use goto button for Align ArgoNavis.
Ps. Yellow buttons Up, Down etc work. Green (GoTo for Align) 50/50 work/not work. GoTo (for GoTo) - works (on both box).